1. Field of the Invention
The present invention relates to a precision machining apparatus and a precision machining method used for working an object which needs to be worked so that the shape/size accuracy and the flatness of a finished surface is high, e.g., a silicon wafer or a magnetic disk substrate. More particularly, the present invention relates to a precision machining apparatus and a precision machining method capable of carrying out grinding with accuracy by performing switching control, for example, on a device for rotating a grinding wheel according to grinding stages through the amount of movement and constant pressure changed stepwise.
2. Background Art
There has recently been an increasing demand for reducing energy loss in next-generation power devices while reducing the size of the devices. An example of such a demand includes a demand for increasing the number of layers in a semiconductor multilayer structure for electronics purposes and increasing the packaging density of semiconductor devices. Examples of methods conceivable as measures to meet such a demand include a method for reducing the thickness of semiconductor wafers typified by a Si wafer to an extremely small value, a working method which prevents dislocation and lattice strain in a worked surface and a portion below a worked surface, and a working method which reduces the surface roughness (Ra) to a value in a range from the subnanometer (nm) level to the nanometer (nm) level and reduces the flatness of a worked surface to a value in a range from the submicrometer (μm) level to the micrometer (μm) level or a lower range.
In the motor vehicle industry, an integrated bipolar transistor (IGBT) which is a power device for motor vehicles forms an essential system in inverter systems. A further improvement in marketability of hybrid vehicles achieved by improving the performance of an inverter using the IGBT and by reducing the size of the inverter is being expected. Reducing the thickness of the Si wafer constituting the IGBT to 50 to an extremely small value of about 150 μm, preferably 80 to 140 μm, more preferably 90 to 120 μm to reduce switching loss, steady loss and thermal loss is indispensable to improving the inverter. Further, an improvement in yield in a process step of forming electrodes on the semiconductor and an increase in the number of layers in the semiconductor multilayer structure can be achieved by forming a perfect surface with no dislocation and no lattice strain in a worked surface of a circular Si wafer having a diameter of 200 to 400 mm or in an internal portion in the vicinity of the worked surface and by reducing the surface roughness (Ra) to a value in a range from the subnanometer level to the nanometer level and the flatness to a value in a range from the submicrometer level to the micrometer level.
In ordinary cases under present circumstances, a multistep process including rough grinding using a diamond grinding wheel, lapping, etching and wet chemo-mechanical polishing (wet-CMP) using a loose abrasive is required for the above-described semiconductor working process. It is extremely difficult to obtain a perfect surface by the conventional working method using such process steps, since an oxide layer, dislocation and lattice strain are produced in the worked surface. Also, the flatness of a wafer worked by the conventional method is low and a break in the wafer may be caused during working or after electrode formation, which leads to a reduction in yield. Further, in the conventional working method, the difficulty in reducing the wafer thickness to an extremely small value is increased with the increase in wafer diameter to 200 mm, to 300 mm and to 400 mm. Studies are presently being conducted to reduce the thickness of a wafer having a diameter of 200 mm to the 100 μm level.
In view of the above-described problem of the conventional art, the inventors of the present invention disclosed an invention relating to a precision surface working machine capable of consistently performing a process from rough working to super-precision surface working including final ductility mode working with efficiency by using only a diamond grinding wheel (JP Patent Publication (Kokai) No. 2000-141207 A).
In this grinding using a diamond grinding wheel, three essential actions: rotation of the grinding wheel, feed by a main spindle supporting the grinding wheel and positioning of an object to be worked are important. These actions are controlled with accuracy to enable precision working. A process from rough working to super-precision working consistently performed by using one apparatus through the entire process, in particular, requires accurate control of feed by the main spindle through a wide range in the above-described essential actions. For example, a system using a servo motor is ordinarily used for control of the main spindle in conventional grinding. However, this system cannot be said to be adequate for accurate control through low-pressure and high-pressure regions. This system is inadequate for working in a low-pressure region in which super-precision working is performed, in particular.
The inventors of the present invention then disclosed a precision working machine in which pressure control is performed by means of a combination of a servo motor and a super-magnetostrictive actuator. Control is performed by means of the servo motor and a piezoelectric actuator in a pressure range of 10 gf/cm2 or higher and is performed by means of the super-magnetostrictive actuator in a pressure range from 0.01 to 10 gf/cm2. In this way, a process from rough working to super-precision working can be consistently performed by using one apparatus through the entire process. In this precision working machine, a diamond cup type of grinding wheel having an abrasive grain size finer than No. 3000.
In the precision working machine disclosed in JP Patent Publication (Kokai) No. 2000-141207 A, a process from rough grinding to super-precision working can be consistently performed by using one apparatus through the entire process, and extremely high accuracy with which a surface to be finished is worked can be achieved. However, there has been a problem that when super-precision working is performed by means of the super-magnetostrictive actuator only, heat generated from the super-magnetostrictive affects other components of the precision working machine and the other components and they may be damaged by the heat.